Purchased a R9300 Solar Geyser from SONKRAG which i cannot use!

This company quoted R9300 for supply and installation of a Solar Geyser to my domestic workers flat. They installed some two weeks ago. The reservoir on top of the geyser overflows 24/7 which forced me to shut the geyser off and revert to Eskom. Initially, they informed me this is due to the fact that we have mixers in use and this causes back pressure. At my own cost [ R1800 ] i replaced all mixers with hot and cold water taps to no avail. They then blamed the water pressure for the problem. They themselves measured the pressure which is low at 3,5Bar [ i am actually considering installing a pressure pump to increase the water pressure to the house ]. They have returned several times but are unable to fix the problem and don't seem to have the know-how to do so. Now they tell me i have to educate my staff on the correct use of the taps - absurd. They are now ignoring me. Clearly their system is inadequate and they should not be in business.